MATERIALS OF HEREDITY 21 



ological action is that causes the difference of sexes will be 

 discussed later. 



Relation of Chromosomes of Parents to Those of Off- 

 spring. — Certain general relations of chromosomes in par- 

 ent and offspring must be held clearly in mind, if one is 

 to understand the further relations of chromosomes to 

 heredity. It will be well to arrange these in a series of 

 propositions: 



/. Every individual gets the chromosomes of its cells 

 half from its mother, half from its father (or in some cases 

 one less than half from its father). 



2. The chromosomes that it gets from one parent are 

 like those that it gets from the other parent, except that 

 from one parent it may get a large X-chromosome, while 

 from the other it may get a small Y-chromosome (or 

 none). 



5. In many organisms, the different chromosomes of the 

 cell are of diverse types, unlike in size and form. Some are 

 round, some are straight, some are V-shaped. This is true 

 in the fruit-fly, for example (figure 7). 



^. One of each type from the mother goes into the ferti- 

 lized egg; also one of each type passes from the father 

 into the fertilized egg. 



5. So the chromosomes of the fertilized egg, and of the 

 individual that it forms, are in pairs, one chromosome of 

 each pair from the father, one from the mother (figure 5). 



6. In some organisms, as in the fruit-fly, the two mem- 

 bers of a pair remain side by side, throughout life, so that 

 we find the chromosomes always in pairs. We know that 

 one member of each such pair came originally from the 

 father, the other from the mother. 



7. In the female, the two members of each pair are alike. 

 The one that came from the mother is exactly similar to 

 the one that came from the father; and this is true for all 

 the pairs. 



